As used herein the term "master device" refers to a programmable device which has the capability both to transmit and receive data and has the capability to initiate a data transaction. It is well known to connect a plurality of such devices together on a single data bus in such a manner that the devices can transmit data to each other or receive data from each other. Passive slave devices may also be connected to the data bus but in general these devices cannot initiate a data transaction.
Various problems may arise in systems where a plurality of master devices are linked together by a single data link. The master devices may try to communicate simultaneously on the data link and thus interfere with each other. Also, in a system where the data provided from one master device has priority over data provided from other master devices, the priority data may be lost because lower priority data is being transmitted on the link at the same time that the higher priority data is ready for transmission.
A data link contention system which has provided an effective solution to the aforementioned problems is disclosed in U.S. Pat. No. 4,359,731, issued Nov. 16, 1982 to Beauford et al. However, the system disclosed in that patent requires both a data link and an access channel. It would be desirable to be able to operate without the access channel.
It is thus an object of this invention to provide method and apparatus for synchronizing access to a data communication link between multiple master devices without requiring an access channel. It is another object of this invention to provide method and apparatus for insuring that high priority data is not lost.